In conventional photolithographic processing, objects are created on a semiconductor wafer by exposing the wafer with light that is passed through a mask or reticle. The mask/reticle has patterns of opaque and clear areas that selectively expose light sensitive chemicals on the wafer. The chemicals are processed to selectively remove portions of a layer, thereby creating the desired structures on the wafer.
As the size of objects being created on a wafer approaches and becomes smaller than the wavelength of light used to expose the wafer, optical distortions can occur such that the pattern of objects defined by the mask or reticle will not match the pattern of objects that are created on the wafer. To improve the pattern fidelity, changes can be made to the mask/reticle patterns that compensate for the expected optical distortions. One common tool for adjusting the mask/reticle patterns is an optical and process control (OPC) tool such as the Calibre® software tools available from Mentor Graphics Corporation of Wilsonville, Oreg., the assignee of the present invention.
As will be appreciated by those skilled in the art, an OPC tool allows a computer to adjust the position of edge segments in polygons that define structures to be created in order to compensate for expected optical distortions. FIG. 1A illustrates a polygon 10 that is divided into a number of edge segments 12, 14, 16, 18, 20, 22, 24, and 26 that extend around the perimeter of the polygon. Each edge segment has a corresponding control site 30a–30h that defines where the edge placement error (EPE) for the corresponding segment should be minimal or zero, and where measurements of process parameters are made. In order to manufacture an object corresponding to the polygon 10 on a wafer, the fragmented polygon is supplied to an OPC software tool that adjusts the position of the edge segments or adds features such as hammerheads or serifs, etc., so that a simulation of the lithographic process indicates that an object will be faithfully created on a wafer (as represented by the curved line 36). In an ideal case, the curved line 36 passes through each of the control sites 30a–30h so that the edge placement error at each of the control sites 30a–30h is minimal.
In some cases, where certain features cannot or can barely be resolved with a particular lithographic process, the placement or presence of certain control sites in a polygon can cause the OPC tool to produce poor or unexpected results. FIG. 1B illustrates an example wherein an OPC tool attempts to adjust the position of the edge segments such that the edge placement error at each control site is minimized. However, because of lithographic process constraints, in order to minimize the edge placement error at the control sites 30a, 30b, 30c, 30d, 30e, 30g, and 30h, the OPC tool causes an object to be created (as represented by the curved line 37) that is far outside the boundaries of the polygon in the area of the control site 30f that is defined for the edge segment 22. Therefore, there is a need for a method of eliminating or adjusting the position of control sites in a fragmented polygon that will prevent or lessen the chance that an OPC tool will produce anomalous results.